a) Field of the invention
The present invention relates to a high-frequency filter and, more particularly, to a high-frequency filter with a patterned multi-layer structure.
b) Description of the Related Art
As electronic equipments miniaturize, their internal elements develop toward having a compacted aggregation. Thus, in order to integrate with other elements, a method for forming filters on planar circuit boards has been made available. FIG. 1A shows a top view of a conventional interdigital planar filter. FIG. 1B shows a sectional view of the filter taken along the line A–A′ as shown in FIG. 1A. As shown in FIGS. 1A and 1B, the conventional interdigital planar filter is formed by patterning substrates with metal materials. The conventional interdigital planar filter includes a metal grounding surface 16, a substrate (such as a printed circuit board) 15, a plurality of resonators 11 formed on the substrate 15, an input port 12, and an output port 13. The resonators 11 are electrically connected to the metal grounding surface 16 via through holes 14 and thereby shorted to ground; the through holes 14 of adjacent resonators are located at opposite ends (interdigitated). Furthermore, the adjacent resonators 11 can couple with each other.
A signal is inputted into the input port 12 and transmitted to the resonator 11 connected thereof. The signal is then transmitted to the resonator 11 connected to the output port 13 through couplings between adjacent resonators 11 sequentially. At last, the signal is sent out from the output port 13. Generally, the distance d between adjacent resonators 11 greatly affects the performance of the interdigital planar filter because it can determine the coupling strength between the two resonators. Common printed circuit boards have a lower dielectric constant (less than 5), which causes their electric field to be more spread out; therefore, if these printed circuit boards are used as substrates to make interdigital planar filters, couplings between adjacent resonators are easier to occur and the distance therebetween is often greater.
Thus, a technology using ceramic materials as substrate has been developed for the purpose of minimizing filter sizes. However, ceramic materials have a very high dielectric constant (usually greater than 7.8). If interdigital planar filters are to be made on ceramic materials, adjacent resonators have to be in extreme proximity in order to accomplish the required coupling strength; the distance d has to be smaller than 100 μm, which is not feasible with the current processing technology. Hence, it is difficult to fabric interditial planar filters on ceramic substrates due to the essential spacing required.
In view of the above problem, a filter that is able to change its structure to accommodate process limitations would solve the problems; at the same time, the filter can be made efficiently on ceramic substrates and the size of the filter can be minimized.